To protect patient organs, in some CT scanning, total scanning dosage values need to be controlled as low as possible. In clinical practices, a factor which varies regularly in a certain stage is generally taken as a trigger condition for realizing a periodical control of the scanning dosage, that is, scanning is performed under an alternation between a scanning mode of low scanning dosage and a scanning mode of high scanning dosage. For example, referring to FIG. 1, during a heart scanning, according to electrocardio signal fluctuations of a patient, a predetermined constant high scanning dosage is used in the scanning during a first time period while a predetermined constant low scanning dosage is used in the scanning during a second time period. For another example, to protect thyroids or eyes during a scanning process, when a radiation tube is facing directly to the thyroids or the eyes, a predetermined constant low scanning dosage should be used in the scanning; and when the radiation tube deflects from the direction straightly towards the thyroids or the eyes, a predetermined constant high scanning dosage is used in the scanning.
However, the inventors found at least following disadvantages in conventional technologies. As described above, a conventional scanning process is performed under an alternation between a scanning mode with low scanning dosage and a scanning mode with high scanning dosage, where a factor which varies regularly is taken as a trigger condition. In the above described conventional process, the total scanning dosage in the scanning is reduced and thus a scanned organ is protected in a manner. However, when scanning different parts of a body or scanning a same part from different scanning angles, attenuation of X-rays is different. Therefore, when a constant high scanning dosage or a constant low scanning dosage is used in scanning, image noises generated from scanning different parts of the body may be inconsistent, and image noises generated from scanning a same part of the body with different scanning angles may be inconsistent as well.